Datasheet

TPS40170

UDG-09202

40.4 µA

11.6 µA

REF

TRK

SS Error

Amplifier

COMP

OUT

Soft-Start

Charge/Discharge

Control

1.05 µA

SS_EAmp

TPS40170

www.ti.com

SLUS970A –MARCH 2011–REVISED NOVEMBER 2013

NOTE

Both OCP and SCP are based on low-side and high-side MOSFET voltage sensing at the

SW node. Excessive ringing on the SW node can have negative impact on the accuracy of

OCP and SCP. Adding an R-C snubber from the SW node to GND helps minimize the

potential impact.

Soft-Start and Fault-Logic

A capacitor from the SS pin to GND defines the SS time, t

. The TPS40170 enters into soft-start immediately

after completion of the overcurrent calibration. The SS pin goes through the device's internal level-shifter circuit

before reaching one of the positive inputs of the error amplifier. The SS pin must reach approximately 0.65 V

before the input to the error amplifier begins to rise above 0 V. To charge the SS pin from 0 V to 0.65 V faster, at

the beginning of the soft-start in addition to the normal charging current, (11.6 µA, typ.), an extra charging current

(40.4 µ A, typ.) is switched-in to the SS pin. As the SS capacitor reaches 0.5 V, the extra charging current is

turned off and only the normal charging current remains. Figure 20 shows the soft-start function block.

Figure 20. Soft-Start Schematic Block

As the SS pin voltage approaches 0.65 V, the positive input to the error amplifier begins to rise (see Figure 21).

The output of the error amplifier (the COMP pin) starts rising. The rate of rise of the COMP voltage is mainly

limited by the feedback loop compensation network. Once V

COMP

reaches the valley of the PWM ramp, the

switching begins. The output is regulated to the error amplifier input through the FB pin in the feedback loop.

Once the FB pin reaches the 600-mV reference voltage, the feedback node is regulated to the reference voltage,

REF

. The SS pin continues to rise and is clamped to VDD.

Product Folder Links :TPS40170